The invention relates to a transmitter and a wireless communication device, which comprises such a transmitter, particularly when a time division duplex TDD and a frequency division duplex FDD are used. Furthermore, the invention is particularly advantageous when the dynamic range of the transmitter's transmission power is wide.
In an FDD, signals are transmitted and received on different frequency bands, whereas in a TDD the same frequency band is used both for transmission and reception and the signals are separated from each other in time. FDD signals are thus typically transmitted continuously and TDD signals are discrete-time signals (bursts). In addition, an FDD transmitter can operate e.g. in compression mode where nothing is transmitted, i.e. transmission is not genuinely continuous. In third generation mobile communication systems, such as the UMTS (Universal Mobile Telecommunication System), both duplexing methods can be employed. For this reason, it is necessary to develop solutions which enable use of both duplexing methods in the same terminal. Seen from the terminal, in the UMTS the FDD transmission band has at the moment a range of 1920 to 1980 MHz and the FDD reception band a range of 2110 to 2170 MHz. Two frequency ranges are reserved for the TDD operation: TDD1 on a frequency band of 1900 to 1920 MHz and TDD1 on a frequency band of 2010 to 2025 MHz. In the future, other frequency bands will also be available for the FDD and TDD functions. In this document we concentrate on describing the structures in connection with the UMTS system, without excluding their applicability to any other system.
In the UMTS standardized transmission power levels vary between −50 and +24 dBm. This variation range of over 70 dB in the power level is needed to restrict interference levels and to eliminate the ‘near-far’ effect. Low transmission power levels are used for transmission of TDD signals, in particular, whereas the transmission power levels of FDD signals are typically higher because the TDD cells are typically micro/pico cells which have a smaller diameter than typical FDD cells, i.e. the maximum distance a TDD signal has to travel is usually shorter than the maximum range required of an FDD signal.
In prior art solutions wide dynamics has been achieved in transmission power control e.g. by first amplifying a signal by an adjustable amplifier and then by a power amplifier to achieve high transmission power levels. For low power levels, the amplification of the amplifier is turned down, but depending on the application, this does not typically reduce the power consumption of the amplifiers. Thus the switching capacity is poor at low power levels, i.e. the method wastes energy, which in turn uses up the power source energy faster. For a mobile user this means a shorter talktime, for example.
FIG. 1 illustrates a prior art transmitter circuit for FDD signals. A modulated signal TX is fed into an adjustable amplifier 1. A signal to be transmitted is obtained from the output of the adjustable amplifier 1 and it is filtered by a filter 2. In this case the filter 2 covers the FDD transmission band of the UMTS, i.e. its frequency band is in a range of 1920 to 1980 MHz. As stated above, this is at the moment a standardized frequency band and in the future it can be replaced by some other frequency band. Next the signal to be transmitted is amplified further by a power amplifier 3. A directional coupler 4 located after the power amplifier 3 takes a sample of transmission power for a detector 5, which generates a DC level comparable to the transmission power, the DC level being further used for controlling the transmission power control. In addition, in the UMTS the network is informed of the detected power level, i.e. the terminal reports the highest power levels it has used to the network (base station). The output of the power amplifier 3 is supplied to a duplex filter 7 via an isolator 6. The duplex filter is a three-port component which comprises separate filters 7a and 7b for the transmitting side and the receiving side and separates transmission and reception signals at different frequencies from each other. The filters 7a and 7b of the transmitting and receiving sides can be tuned to a desired frequency range. In other words, the transmitting side 7a of the duplex filter is tuned to a frequency range of 1920 to 1980 MHZ. The structure of the receiving side is not discussed here, neither is it described in the figure.
The antenna port 7c of the duplex filter is connected to an antenna coupler 8 for connecting the transmission branch 12 described above or a transmission branch 13 adapted to another mobile communication systems, such as the GSM (Global System for Mobile Communication) to an antenna 9. This structure does not imply how the TDD operation is implemented. TDD operation could be simply provided with a transmission branch of its own. To minimize the number of components, it would be advantageous to combine TDD and FDD signals into the same transmission branch in a terminal, particularly in the UMTS system where the TDD and FDD operations have been harmonized to correspond to each other to as large an extent as possible.
U.S. publications No. 5,152,004, Procedure for Forming Lower Power Levels in a Radio Telephone Transmitter, Väisänen et al., and U.S. Pat. No. 5,909,643, Transmitter power varying device having a bypass line for a power amplifier, Aihara, disclose solutions for generating low power levels by bypassing the power amplifier at low power levels. These solutions do not, however, provide a solution for transmitting signals of different types using the same signal path.